ox

From Middle English oxe, from Old English oxa, tracing back through Proto-West Germanic *ohsō and Proto-Germanic *uhsô to Proto-Indo-European *uksḗn. In its other life, it's simply a clipping of oxygen.

The animal name has stayed remarkably steady across millennia, passing down through Germanic languages largely intact in meaning. The clipped sense, used for oxygen, is a much more recent and entirely separate development.

It's cognate with Welsh ych, Tocharian A ops and Tocharian B okso, Avestan uxšan, and Sanskrit ukṣán, all pointing back to a shared Proto-Indo-European word for this animal.